The present invention relates to a photovoltaic element, such as a solar cell and an optical sensor, with a structure comprising a transparent conductive film deposited on a semiconductor layer like a pin junction, and also relates to a photovoltaic device using the photovoltaic element.
In recent years, the introduction of a solar power generation system that is installed on the roof of a house has been promoted rapidly. A solar cell for use in the solar power generation system utilizes, for example, a HIT (heterojunction with intrinsic thin-layer) type photovoltaic element which is produced by stacking i-type and p-type amorphous semiconductor layers and a transparent conductive film made of an Sn-doped indium oxide film (hereinafter referred to as an ITO (Indium Tin Oxide) film) successively on an n-type crystalline silicon wafer and forming a collecting electrode on the transparent conductive film, or a photovoltaic element which is obtained by forming a back electrode, n-type, i-type and p-type amorphous semiconductor layers, a transparent conductive film made of an ITO film, and a collecting electrode in this order on a substrate having an insulating surface such as glass plate, plastic plate, or metal plate with an insulating film formed on the surface thereof.
Since a solar cell module using such a photovoltaic element is generally mounted outdoors, it needs to have high environment resistance reliability. Hence, conventionally, when a photovoltaic element is incorporated as a product into a module, a cover glass is often used for the purpose of protecting the photovoltaic element, and thereby ensuring environmental resistance of the module.
As the cover glass, in general, inexpensive soda glass is used. However, under conditions such as a high humidity condition, alkaline ions such as Na, Li and K contained in the soda glass may be diffused into the transparent conductive film and amorphous semiconductor layers, and may give a vicious influence on the transparent conductive film and amorphous semiconductor layers. If alkaline ions are diffused into the transparent conductive film, the conductivity decreases, and abnormalities in the refractive index, etc. occur. If alkaline ions are diffused into the amorphous semiconductor layers, a diffusion potential change occurs, and there arises a problem of deterioration of the characteristics of the photovoltaic element.
It is therefore desirable that the photovoltaic element itself should have excellent environment resistance, particularly excellent resistance against alkaline ions, and there has been demand for an improved transparent conductive film. Moreover, the transparent conductive film of the photovoltaic element is required to have high light transmittance and low electrical resistance for high efficiency. In general, in order to achieve high light transmittance and low electrical resistance, it is necessary to improve the crystallinity of the transparent conductive film. In this case, however, in the ITO that is a polycrystalline substance, the crystal grains become larger, and accordingly the influence of the crystal grain boundaries increases. Thus, there are possibilities of promotion of diffusion of alkaline ions through the grain boundaries as the path and a decrease in the environment resistance reliability.
As a method for preventing the diffusion of alkaline ions, it is considered to provide a diffusion preventing layer (for example, SiO2 layer) against the alkaline ions, between the cover glass and the transparent conductive film. However, this method has problems that the additional step of forming the diffusion preventing layer and extra cost are required.